In order to obtain Streptococcus thermophilus with high yield of conjugated linoleic acid (CLA) to develop functional fermented milk, effects of glycine concentration, bacterial density, buffer solution, voltage and plasmid concentration on electroporation efficiency of S. thermophilus were investigated. The growth of S. thermophiles was negatively correlated with glycine concentration, and its electroporation efficiency was the highest when the cells grew with 10 g/L glycine. The efficiency increased and then decreased with increasing cell density, transformation voltage, and plasmid concentration. The optimal conditions for electroporation were: cell density of 0.8 (OD600), voltage of 1.8 kV, and plasmid concentration of 1.0 g/L. Moreover, buffer II (pH=6.0) composed of 0.5 mol/L sucrose and 1 mmol/L ammonium citrate showed good transformation efficiency. By optimizing electrotransformation conditions, the transformation rate of S. thermophiles increased, which provides a theoretical basis for transforming lactic acid bacteria efficiently and also gives a new idea for producing fermented milk with CLA.
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